CMOS (Complementary Metal Oxide Semiconductor) image sensors having a large chip size and having a large number of pixels of more than ten million have conventionally existed. In such CMOS image sensors, an area to be exposed at a time is limited in a manufacturing process, and thus stitching exposure is performed in twice for the left and right in an ordinary case. At the time, due to variations in manufacturing conditions, it is very difficult to form left and right regions so that they are exactly the same as each other. Accordingly, a difference in dark current occurs between left and right pixel circuits, and as a result, an offset may occur between left and right images.
For example, Patent Document 1 discloses a technique of performing clamp correction by using the following both signals (1) and (2) in order to eliminate an offset difference that occurs between the left and right of an imaging region of a CCD (Charge Coupled Device) or a CMOS image sensor of a CDS (Correlated Double Sampling) method. Signal (1) is a signal of a left/right dummy pixel (a signal independent of a photoelectric converting unit in extra HCCD is adopted). Signal (2) is a signal of a left/right OB pixel (optical black pixel: a light-shielded pixel in which a signal dependent on a photoelectric converting unit is adopted).
However, in the technique according to Patent Document 1, signals (1) and (2) are required to be subtracted from pixel signals in an effective pixel region. Also required are two systems of circuits for the left and right to perform CDS, A/D conversion, OB clamp, and the like. Accordingly, a circuit configuration is complicated and a circuit area increases.
For example, Patent Document 2 discloses a technique of maintaining an OPB (Optical Black) level of an imaging device as a DC level of an image signal at a constant value in accordance with a reference level in a CMOS image sensor of a current output method of outputting a pixel signal as a current signal. Patent Document 2 describes an embodiment in which an OPB region is placed next to a photo sensor area region. Furthermore, Patent Document 2 discloses a technique of feeding back a clamp current to an image signal so that a difference between a detected output level and a predetermined reference voltage value becomes substantially zero.
For example, Patent Document 3 discloses a technique using the following black level correcting method and gain correcting method as a technique for a solid-state imaging device including a CCD. In the black level correcting method, pixel information for a screen read in units of lines is output while being divided for a plurality of channels. The black level of the pixel information read from an image sensor is detected in each channel, and the black level of the pixel information is corrected in each channel. On the other hand, in the gain correcting method, a gain difference between channels of pixel information read from the image sensor is detected, and the gain difference between the channels is corrected. Also, Patent Document 3 discloses a technique capable of performing black level correction independently for left and right channels.
However, in the technique described in Patent Document 3, two channels on the left and right are necessary for output from a horizontal register.
Also, for example, Patent Document 4 discloses the following technique. That is, an OB (OPB) area is provided on both left and right sides of an effective pixel region, which is divided into a plurality of regions. First and second OB signals indicating an optical black level are output from first and second OB areas from image signals output from a plurality of split imaging areas of a solid-state imaging device. Any of those OB signals is subtracted, whereby OB clamp of image signals is performed. Also, Patent Document 4 discloses a technique of calculating an average value of OB signals in respective blocks in each OB area.
However, two channels on the left and right are necessary also in the technique according to Patent Document 4. Furthermore, the technique according to Patent Document 4 adopts a method for subtracting an optical black level in an OB (OPB) region from image signals output from respective split imaging areas on the left and right. Therefore, in the technique according to Patent Document 4, an imaging process in the imaging area is necessary, and the time for calculating a correction value is taken.
In the two-channel method (multi-channel method) adopted in the techniques according to the above-mentioned Patent Documents 1, 3, 4, and the like, a parallel process is performed in a clamp correction process, and thus power consumption cannot be reduced although the processing speed increases. In addition, an image combining process circuit for combining pieces of image data of two channels (multi-channels) into a piece of image data after the correction process is necessary. Furthermore, process circuit elements on the left and right increase due to the two channels for reducing a left/right offset. Accordingly, circuit errors increase due to variations in manufacturing conditions including wiring, so that an offset increases.
Also, as conventional clamp methods used in a CMOS image sensor, there exist a vertical clamp method using vertical OPB and a horizontal clamp method using horizontal OPB. Particularly, in the horizontal clamp method, correction is uniformly performed on the entire effective pixel region for each line even if an OPB value differs in left and right screens. Therefore, the horizontal clamp method is inappropriate for clamp control of performing the above-described stitching exposure.    Patent Document 1: Japanese Patent No. 3697172    Patent Document 2: Japanese Patent No. 3969190    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-252808    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2007-6538